This can happend for different reasons. Here are some things you should check (in prioritized order):
1. permission denied. Apache/PHP does not have read access to these files. Check the read permissions to the files and make sure that the apache user has accesse to read them.
2. include path does not include . Verify that your include path settings in PHP include . ( the current directory ).
3. the file is not there. Verify that the file is actually where PHP looks for it.